Laser absorption spectroscopy that utilizes tunable lasers as the light source is useful in measuring the absorption of samples. In the simplest measurement apparatus the incident light is caused to pass through the sample and is detected by an optical detector. For the case of a strong absorber, the signal at the detector can be small and difficult to measure. Furthermore, the described simple measurement fails to measure the dispersion of the sample.
Heterodyne-based detection systems provide significant improvements over the simple system discussed above. In a heterodyne detection system, the output of the laser is modulated at an RF frequency to create a light component at a slightly different wavelength that provides a reference signal that is mixed with the signal from the sample. This reference signal gives rise to a beat frequency in the detector output. A strong reference signal can generate a beat signal which is easier to detect for instances where the absorption is strong. The beat signal also provides both amplitude and phase information that can be used to provide a measurement of the absorption and dispersion introduced by the sample.
Heterodyne-based receivers that utilize an acoustic optical modulator to generate the second frequency are known to the art. However, these modulators add complexity to the instrument and are limited in the wavelengths that can be generated for the reference beam without repositioning the acoustic optical modulator.